@article {300, title = {Solvent polarity and hydrogen-bonding effects on the nitrogen NMR shieldings of N-nitrosamines and DFT calculations of the shieldings of C-, N-, and O-nitroso systems}, journal = {Journal of Magnetic Resonance}, volume = {164}, year = {2003}, month = {2003 Oct}, pages = {212-9}, abstract = {High-precision nitrogen NMR shieldings, bulk susceptibility corrected, are reported for dimethyl-N-nitrosamine (I) and diethyl-N-nitrosamine (II) in a variety of solvents which represent a wide range of solvent properties from the point of view of polarity as well as hydrogen bond donor and acceptor strength. The observed range of solvent-induced nitrogen shielding variations of (I) and (II) is significant for the amino-type nitrogens, up to about 16 ppm, and originates essentially from the deshielding effect of the increasing polarity of solvent. On the other side, the nitroso nitrogen shieldings reveal an even stronger response to solvent effects, within about 20 ppm, but in this case the increasing polarity and hydrogen bond donor strength of solvent produce enhanced shielding. DFT quantum-mechanical calculations using the GIAO/B3PW91/6-311++G** approach and geometry optimizations employing the same basis set and hybrid density functionals show an excellent correlation with the experimental data on C-, N-, and O-nitroso moieties and reproduce not only major changes but also most of the subtle variations in the experimental nitrogen shieldings of the nitroso systems as a whole. A combination of the calculations involving the corresponding N and O-protonated species and the trends observed in the solvent-induced nitrogen shielding variations shows clearly that the prime acceptor site for hydrogen bonding is the nitroso oxygen atom.}, keywords = {Algorithms, Binding Sites, Carbon, Computer Simulation, Hydrogen Bonding, Macromolecular Substances, Magnetic Resonance Spectroscopy, Models, Molecular, Molecular Conformation, Molecular Structure, Nitrogen, Nitrogen Isotopes, Nitrosamines, Nitroso Compounds, Oxygen, solutions, Solvents}, issn = {1090-7807}, author = {Michal Witanowski and Zenobia Biedrzycka and Wanda Sicinska and Zbigniew Grabowski} } @article {301, title = {A study of solvent polarity and hydrogen bonding effects on the nitrogen NMR shieldings of N-nitramines and ab initio calculations of the nitrogen shieldings of C-nitro, N-nitro and O-nitro systems}, journal = {Journal of Molecular Structure}, volume = {602{\textendash}603}, year = {2002}, pages = {199 - 207}, abstract = {High-precision nitrogen NMR shieldings, bulk susceptibility corrected, are reported for dimethylnitramine (1) and diethylnitramine (2) in a variety of solvents which represent a wide range of solvent properties from the point of view of polarity as well as hydrogen bond donor and acceptor strength. The observed range of solvent-induced nitrogen shielding variations of 1 and 2 is significant for the amino-type nitrogens, up to about 10 ppm, and originates essentially from the deshielding effect of the increasing polarity of solvent. On the other side, the nitro nitrogens reveal a very weak and rather chaotic response to solvent effects, within about 2 ppm. This is in a sharp contrast with the behavior of the nitrogen shieldings of C-nitro and O-nitro groups where solvent effects induce variations within at least 6 ppm which follow a regular pattern of enhanced magnetic deshielding with increasing polarity involved. This apparent insensitivity of the N-nitro group shielding to solvent effects seems to stem from the cancellation of opposite effects, the shielding effect of the electron charge migration to the nitro group upon increasing solvent polarity, and the deshielding effect produced by the flattening of the pyramidal arrangement of bonds at the amino nitrogen with the increasing polarity of the medium. Ab initio quantum-mechanical calculations using the View the MathML source approach and geometry optimizations employing the same basis set and density functionals show an excellent linear correlation with the experimental data and reproduce not only major changes but also most of the subtle variations in the experimental nitrogen shieldings of the nitro systems as a whole.}, keywords = {Ab initio calculations, Hydrogen Bonding, N-nitramines, Nitrogen NMR shieldings, Solvent effects}, issn = {0022-2860}, doi = {http://dx.doi.org/10.1016/S0022-2860(01)00734-7}, url = {http://www.sciencedirect.com/science/article/pii/S0022286001007347}, author = {Michal Witanowski and Zenobia Biedrzycka and Wanda Sicinska and Zbigniew Grabowski} } @article {304, title = {Nitrogen NMR shieldings of thiourea systems as a function of solvent polarity and hydrogen bond effects}, journal = {Journal of Molecular Structure}, volume = {516}, year = {2000}, pages = {107 - 112}, abstract = {Tetramethylthiourea, 1, and thiourea, 2, have been studied in a wide variety of solvents by high-precision 14N NMR measurements. The chosen solvents exhibit a wide range of hydrogen-bonding and polarity/polarisability properties. The observed nitrogen shielding variations of the solutes, due to solvent change, are significant and are attributed to solvent polarity, solute to solvent and solvent to solute hydrogen-bonding effects. Nitrogen deshielding of 1 and 2 is produced by all these interactions, due to an enhanced delocalisation of the nitrogen lone-pair electron into the π systems of 1 and 2. The nitrogen NMR shieldings of 1 and 2 are calculated by the CHF{\textendash}GIAO ab initio molecular orbital procedure using a 6-31++G** basis set. The calculations are for isolated molecules, and their results satisfactorily reproduce the position of the thiourea nitrogen resonance obtained in a dilute solution in cyclohexane with respect to that of urea systems and nitromethane.}, keywords = {14N NMR, Ab initio molecular orbital calculations, Hydrogen-bonding, Solvent polarity, Thiourea systems}, issn = {0022-2860}, doi = {http://dx.doi.org/10.1016/S0022-2860(99)00202-1}, url = {http://www.sciencedirect.com/science/article/pii/S0022286099002021}, author = {Michal Witanowski and Zenobia Biedrzycka and Wanda Sicinska and Graham A. Webb} } @article {303, title = {Study of hydrogen bonding and solvent polarity effects on the nitrogen NMR shieldings of N,N-dimethylacetamidine}, journal = {Magnetic Resonance in Chemistry}, volume = {38}, year = {2000}, pages = {177-182}, abstract = {Solvent effects on the nitrogen shieldings of N,N-dimethylacetamidine (1) were found to be extremely large for the imino group (about 120 ppm) and for the amino moiety (about 50 ppm). A detailed analysis of the solvent-induced variations revealed contributions from three large interactions. These are due to solvent polarity, hydrogen bonding from solute to solvent, via the NH moiety of the solute, and from solvent to solute, involving the nitrogen lone pairs of the solute. For the imino moiety, large shielding effects are observed due to solvent polarity and solvent-to-solute hydrogen bonding, whereas solute-to solvent hydrogen bonding leads to a large decrease in nitrogen shielding. For the NMe2 group, the changes are of opposite sign and smaller in magnitude. Ab initio CHF-GIAO magnetic shielding calculations employing a 6{\textendash}31++G** basis set and geometries optimized using the same basis set are reported for 1 and some related molecules. The experimental range of nitrogen shielding considered is about 270 ppm and shows an excellent linear correlation with the calculated results. The least-squares standard deviation amounts to only 1.7\% of the observed shielding range. The effects on the nitrogen shielding of 1 caused by full protonation are accurately reproduced within this correlation. Copyright {\textcopyright} 2000 John Wiley \& Sons, Ltd.}, doi = {10.1002/(SICI)1097-458X(200003)38:3<177::AID-MRC621>3.0.CO;2-I}, url = {http://dx.doi.org/10.1002/(SICI)1097-458X(200003)38:3<177::AID-MRC621>3.0.CO;2-I}, author = {Michal Witanowski and Wanda Sicinska and Zenobia Biedrzycka and Graham A. Webb} } @article {305, title = {Hydrogen-bond and solvent polarity effects on the nitrogen NMR shielding of urea systems}, journal = {Journal of Molecular Structure}, volume = {476}, year = {1999}, pages = {133 - 138}, abstract = {High precision 14N NMR measurements of the nitrogen shieldings of tetramethylurea, 1, and urea, 2, are reported for dilute solutions in a set of solvents which represent a wide spectrum of properties from the point of view of hydrogen-bonding and solvent polarity/polarisability. It is shown that solvent induced nitrogen shielding changes are appreciable and can be attributed to solute-to-solvent, solvent-to-solute hydrogen-bonding and solvent polarity effects. All of these interactions produce deshielding of the nitrogen nuclei of 1 and 2. This is in accord with an enhanced delocalisation of the nitrogen lone-pair electrons into the π systems of 1 and 2 due to the solute{\textendash}solvent interactions. GIAO-CHF ab Initio molecular orbital calculations, using a 6-31++G** basis set, are reported for the nitrogen shieldings of the isolated molecules 1 and 2. These reproduce quite well the position of the urea nitrogen resonance obtained in a dilute solution in cyclohexane, with respect to some model compounds which include amino and amido moieties.}, keywords = {Hydrogen-bonding, Nitrogen Shieldings, Solvent polarity effects, Tetramethylurea, Urea}, issn = {0022-2860}, doi = {http://dx.doi.org/10.1016/S0022-2860(98)00540-7}, url = {http://www.sciencedirect.com/science/article/pii/S0022286098005407}, author = {Michal Witanowski and Wanda Sicinska and Zenobia Biedrzycka and Graham A. Webb} } @article {306, title = {A Study of Solvent Polarity and Hydrogen Bonding Effects on the Nitrogen NMR Shielding of Isomeric Tetrazoles andab InitioCalculation of the Nitrogen Shielding of Azole Systems}, journal = {Journal of Magnetic Resonance}, volume = {131}, year = {1998}, pages = {54 - 60}, abstract = {High-precision nitrogen NMR shieldings, bulk susceptibility corrected, are reported for theN-methyl derivatives of the two existing isomeric tetrazoles (I, II) in a variety of solvents which represent a wide range of solvent properties from the point of view of polarity as well as hydrogen bond donor and acceptor strength. The observed range of solvent-induced nitrogen shielding variations of I and {\.I}I\} is significant for the pyrrole-type nitrogens (N-Me), up to 9 ppm, and even more so for pyridine-type nitrogen atoms, where it can attain a value of 20 ppm. There is a clear distinction between the two types of nitrogen atoms in that the former exhibit a deshielding effect with increasing polarity of the medium while the latter experience an increase in the magnetic shielding of their nuclei. The latter effect is significantly augmented by solvent-to-solute hydrogen-bond formation where the pyridine-type nitrogens are involved directly. It is also quite diversified throughout the pyridine-type nitrogen atoms and seems to constitute a measure of relative basicity with respect to hydrogen-bond formation of the nitrogens concerned. This basicity seems to parallel that with respect to a full transfer of a proton, as can be reckoned fromab initiocalculations of the relevant protonation energies reported in the present study. The experimental data for the tetrazoles in cyclohexane solutions are combined with those obtained in our earlier extensive studies on azole, diazole, and triazole ring systems, for a comparison withab initiocalculations of the nitrogen shieldings concerned. The latter were carried out using the coupled Hartree{\textendash}Fock/GIAO/6-31++G** approach and geometry optimizations employing the same basis set; they show a good linear correlation with the experimental data and reproduce not only major changes but also most of the subtle variations in the experimental nitrogen shieldings of the azole systems as a whole.}, keywords = {azoles, calculations, diazoles, nitrogen chemical shifts, nitrogen NMR, Nitrogen Shieldings, Solvent effects, tetrazoles, triazoles}, issn = {1090-7807}, doi = {http://dx.doi.org/10.1006/jmre.1997.1325}, url = {http://www.sciencedirect.com/science/article/pii/S1090780797913252}, author = {Michal Witanowski and Zenobia Biedrzycka and Wanda Sicinska and Zbigniew Grabowski} } @article {307, title = {Hydrogen Bonding and Solvent Polarity Effects on the Nitrogen NMR Shielding of 1,2,4,5-Tetrazine}, journal = {Journal of Magnetic Resonance}, volume = {124}, year = {1997}, pages = {127 - 131}, abstract = {Results of the first extensive investigation of the nitrogen shielding of 1,2,4,5-tetrazine (I) are reported. Due attention is paid to solvent-induced nitrogen-shielding variations by a variety of solvents. A comparison is made with comparable, earlier, results for the other azine systems. The observed range of solvent-induced nitrogen-shielding variations ofIis modest but significant, about 10 ppm. The major factor producing this variation is that due to solvent polarity effects, which is supported by the results of some shielding calculations based upon the Solvaton model. A smaller, but significant, contribution is produced by solvent-to-solute hydrogen bonding effects. This correlates well with the low nitrogen basicity ofIas indicated by publishedab initiomolecular orbital calculations of gas-phase protonation energies. There is found a general correlation between nitrogen basicity with respect to hydrogen bonding and that with respect to protonation. GIAO/CHF nitrogen-shielding calculations are reported for all of the existing monocyclic heteroaromatic azine systems includingIusing a 6-31++G** basis set. These appear to indicate that the inclusion of electron-correlation effects, reported for the \{SOLO\} method, does not result in a dramatic improvement in the calculated nitrogen shieldings.}, keywords = {Solvent polarity effects}, issn = {1090-7807}, doi = {http://dx.doi.org/10.1006/jmre.1996.1007}, url = {http://www.sciencedirect.com/science/article/pii/S1090780796910071}, author = {Michal Witanowski and Zenobia Biedrzycka and Wanda Sicinska and Zbigniew Grabowski and Graham A. Webb} } @article {341, title = {Nitrogen NMR shieldings of 2-amino-5-nitro-6-methylpyridines}, journal = {International Journal of Spectroscopy}, volume = {13}, year = {1997}, pages = {251-256}, author = {B. Palasek and A. Puszko and Zenobia Biedrzycka and Wanda Sicinska and Michal Witanowski} } @article {308, title = {Solvent and protonation effects on nitrogen NMR shieldings of isoamides (α-aminoethers)}, journal = {Journal of Molecular Structure}, volume = {404}, year = {1997}, pages = {267 - 271}, abstract = {High-precision solvent-induced 14N \{NMR\} shieldings are presented for some isoamides and their N-protonated forms. The results indicate that the major source of solute-solvent interactions, giving rise to nitrogen shielding variations, is solvent-to-solute hydrogen bonding where the lone pair electrons of the imino nitrogen act as the hydrogen-bond acceptor centre. This results in a remarkable shielding increase for the imino nitrogen atom. Another interaction which could be important, when \{NH\} moieties are present, is solute-to-solvent hydrogen bonding where the \{NH\} croup is engaged as a donor. In the present study this effect is found to be a modest one and produces nitrogen deshielding. The influence of non-specific interactions, arising from solvent polarity effects, is rather feeble compared with that for analogous imine systems. The direction and magnitude of this effect is correctly reproduced by molecular orbital calculations incorporating the Solvaton model for non-specific solute-solvent interactions. Protonation effects on the nitrogen shieldings of the isoamides are substantial, producing an increase in shielding of about 80 ppm. This is in accord with that observed for solvent-to-solute hydrogen bonding.}, keywords = {Hydrogen Bonding, Isoamide (α-imino ether), Nitrogen NMR shieldings, Protonation effects, Solvent effects}, issn = {0022-2860}, doi = {http://dx.doi.org/10.1016/S0022-2860(96)09530-0}, url = {http://www.sciencedirect.com/science/article/pii/S0022286096095300}, author = {Michal Witanowski and Wanda Sicinska and Zenobia Biedrzycka and Graham A. Webb} } @article {310, title = {Solvent induced variations in nitrogen NMR shieldings of some oxime systems as a test of the Solvaton model of non-specific molecular interactions}, journal = {J. Chem. Soc.{,} Perkin Trans. 2}, year = {1997}, pages = {533-536}, publisher = {The Royal Society of Chemistry}, abstract = {High precision N NMR measurements are reported for three oxime systems in a variety of solvents. Both hydrogen bonding and solvent polarity effects are found to make significant contributions to the observed range of nitrogen shieldings. For a non-conjugated oxime moiety the solvent induced nitrogen shielding variations are found to be of opposite sign to those found for oximes which have a strongly conjugated carbonyl group. These shielding variations are interpreted in terms of solute-solvent specific and non-specific interactions. The analysis of the shielding data results in the view that{,} in these two cases of carbonyl group conjugation{,} there are opposing directions of electronic charge migration as a function of an increase in solvent polarity. There appears to be a change in the preference of the solute hydrogen bond acceptor site from the oxime to the carbonyl moiety in the case of p-benoquinone mono oxime derivatives. Medium polarity effects on the solute nitrogen shielding are adequately accounted for by INDO/S parametrised molecular orbital calculations of nitrogen shieldings as a function of solvent relative permittivity (I[small micro]) by means of the Solvaton model. This model describes satisfactorily the effects of a change in solvent polarity on the nitrogen shielding of a variety of nitrogen containing solutes.}, doi = {10.1039/A606216D}, url = {http://dx.doi.org/10.1039/A606216D}, author = {Michal Witanowski and Zenobia Biedrzycka and Wanda Sicinska and Graham A. Webb} } @article {309, title = {Solvent-Induced Effects on the Nitrogen NMR Shieldings of Some Nitrosobenzene Systems}, journal = {Magnetic Resonance in Chemistry}, volume = {35}, year = {1997}, pages = {262{\textendash}266}, publisher = {John Wiley \& Sons, Ltd.}, abstract = {High-precision14N NMR measurements of solvent-induced shielding variations are reported for some nitrosobenzene systems. These variations are shown to result from a combination of three major factors, solvent to solute hydrogen bonding where the solute nitrogen lone pair electrons are involved, solvent polarity and interactions between the electron-deficient benzene ring of the nitrosobenzenes and basic centres in the solvent molecules. The last of these three factors produces nitrogen deshielding of the nitroso group, and in the present work this interaction was found to be the largest of its type so far observed. Consequently, this implies that, in nitroso aromatic compounds, the benzene ring shows a remarkable deficit of electronic charge. The former two factors produce an increase in nitroso nitrogen shielding, thus indicating a strong electron-withdrawing effect of the nitroso group which is consistent with previous observations. INDO/S parameterized molecular orbital calculations of solute nitrogen shieldings, incorporating the Solvaton model of non-specific solute{\textendash}solvent interactions, predict that the nitrogen shielding will increase as the polarity of the medium increases. This is in very good agreement with the observation that the nitroso nitrogen shielding analysis yields a large and positive value for thesterm which describes the influence of solvent polarity/polarizability on the shielding variation as a function of solvent. For the 0.2 M solutions studied at 35{\textdegree}C, a significant amount of the dimeric isodioxy form is only observed foro-nitrosotoluene. {\textcopyright} 1997 by John Wiley \& Sons, Ltd}, keywords = {14N, calculations, Nitroso Compounds, NMR, Solvent effects}, issn = {1097-458X}, doi = {10.1002/(SICI)1097-458X(199704)35:4<262::AID-OMR96>3.0.CO;2-5}, url = {http://dx.doi.org/10.1002/(SICI)1097-458X(199704)35:4<262::AID-OMR96>3.0.CO;2-5}, author = {Michal Witanowski and Zenobia Biedrzycka and Wanda Sicinska and Graham A. Webb} } @article {317, title = {Solvent effects on nitrogen NMR shieldings in thiazole and thiadiazole systems}, journal = { Journal of the Chemical Society, Perkin Transactions 2 }, year = {1996}, pages = {619-623}, publisher = {The Royal Society of Chemistry}, abstract = {High precision N NMR measurements are reported for all six possible thiazole and thiadiazole molecules in a variety of solvents. Both solvent polarity and hydrogen bonding effects on the nitrogen nuclear shielding of the solutes are significant. Both types of effect produce an increase in the solute nitrogen shielding. Analysis of the experimental data and molecular orbital studies indicate that an increase in the polarity of the solvent favours the delocalization of the lone pair electrons from the sulfur atoms into the conjugated rings. This leads to an increase in electronic charge at the nitrogen atom(s) concerned. This effect is more pronounced than analogous effects observed for pyridine type nitrogen atoms in the corresponding diazole and triazole systems. The significant shielding effects which result from solvent to solute hydrogen bonding to ring nitrogen atoms are shown to be essentially local in origin. Thus the shielding concerned is that of the nitrogen atom directly involved in the hydrogen bonding.}, doi = {10.1039/P29960000619}, url = {http://dx.doi.org/10.1039/P29960000619}, author = {Michal Witanowski and Wanda Sicinska and Zenobia Biedrzycka and Zbigniew Grabowski and Graham A. Webb} } @article {316, title = {Solvent Effects on the Nitrogen NMR Shieldings in Oxazole and Oxadiazole Systems}, journal = {Journal of Magnetic Resonance, Series A}, volume = {120}, year = {1996}, pages = {148 - 154}, abstract = {High-precision14N NMR shieldings are reported for all five available oxazoles and oxadiazoles in a variety of solvents. Both solvent polarity and hydrogen-bond effects on the nitrogen nuclear shieldings of the solutes are significant and comparable in magnitude; both give rise to shielding increases. The increasing solvent polarity favors delocalization of electrons from oxygen atoms into the heteroaromatic rings with a concomitant electron charge accumulation on the nitrogen atoms concerned. This trend is parallel to those found for analogous diazole and triazole heteroaromatic systems. Solvent-to-solute hydrogen bonding causes an increase in nitrogen shielding when the nitrogen atoms in question are involved in the hydrogen bonding as acceptor sites. This reflects not only local effects of hydrogen bonding to a particular nitrogen atom but also those arising from competition between other acceptor sites in a given molecule; these include oxygen and other nitrogen atoms. TNDO/2-calculated nitrogen shieldings, with respect to neat nitromethane, give a very good linear correlation with experimental results when all of the nitrogen atoms in the molecules studied are taken into account. The present study of nitrogen shieldings provides a valuable insight into solvent polarity effects on electric charge distribution in nitrogenous heteroaromatic systems, as well as into possible competition between various hydrogen-bond acceptor centers with respect to hydrogen-bond donor solvents.}, issn = {1064-1858}, doi = {http://dx.doi.org/10.1006/jmra.1996.0112}, url = {http://www.sciencedirect.com/science/article/pii/S1064185896901126}, author = {Michal Witanowski and Zenobia Biedrzycka and Wanda Sicinska and Zbigniew Grabowski and Graham A. Webb} } @article {315, title = {Solvent effects on the nitrogen NMR shieldings of cyanamide and N,N-dimethyl cyanamide}, journal = {Journal of Molecular Structure}, volume = {380}, year = {1996}, pages = {133-137}, abstract = {High precision N NMR measurements of solvent induced nitrogen shielding variations in both the NR2 and cyano moieties of cyanamide and of N,N-dimethylcyanamide are reported. The solvent induced shielding changes are much larger for the cyano, than for the NR2 groups. The NMe2 group has a nitrogen shielding which is almost invariant to choice of solvent. Solvent effects on the cyano groups of the compounds studied closely match those observed for simple alkyl cyanides. Overall analysis of the nitrogen shielding data favours a highly non-planar geometry for the compounds studied and no significant π electron conjugation between the NR2 and the cyano groups. }, keywords = {Cyanamide, N, N-dimethyl cyanamide, NMR spectroscopy, Solvent effects}, doi = {doi:10.1016/0022-2860(95)09205-6}, url = {http://www.ingentaconnect.com/content/els/00222860/1996/00000380/00000001/art09205}, author = {Michal Witanowski and Wanda Sicinska and Zenobia Biedrzycka and Graham A. Webb} } @article {318, title = {Assessment of Solvent-Induced Nitrogen Shielding Variations of Triazole Systems}, journal = {Journal of Magnetic Resonance, Series A}, volume = {112}, year = {1995}, pages = {66 - 71}, abstract = {High-precision 14N \{NMR\} shieldings are reported for all of the possible N-methyl triazoles in a variety of solvents. A large difference is observed in the effects of solvent on the shieldings of the pyrrole-type and pyridine-type nitrogen atoms in such systems. This difference largely arises from solvent-to-solute hydrogen-bonding effects for the latter type of nitrogen atom. For these two types of nitrogen atoms in triazoles, solvent polarity effects induce changes in the shieldings in opposite directions: this is corroborated by shielding calculations using the solvaton model. Solvent-to-solute hydrogen-bonding effects are larger than those due to solvent polarity and permit the assessment of the basicities of nonequivalent nitrogenous sites; these appear to parallel the analogous basicities with respect to protonation.}, issn = {1064-1858}, doi = {http://dx.doi.org/10.1006/jmra.1995.1010}, url = {http://www.sciencedirect.com/science/article/pii/S1064185885710108}, author = {Michal Witanowski and Wanda Sicinska and Zenobia Biedrzycka and Zbigniew Grabowski and Graham A. Webb} } @article {319, title = {Solvent induced nitrogen NMR shielding variations of some covalent isocyanates}, journal = {Journal of Molecular Structure}, volume = {328}, year = {1994}, pages = {253 - 257}, abstract = {Investigations of solvent-induced nitrogen shielding variations for three covalent isocyanates show that the small changes observed are governed mainly by solvent polarity effects. A less significant contribution arises from solvent to solute hydrogen bonding, since the two contributions are of opposite signs, the overall range of solvent induced changes in the nitrogen shieldings of covalent isocyanates is relatively small compared with those of C=N moieties contained in imino-type structures or heteroaromatic rings. Solvaton model nitrogen shielding calculations support the observed signs of the solvent polarity effects exhibited by the nitrogen shieldings of these groups of molecules. Analysis of the observed nitrogen shielding variations with respect to a change of solvent shows that oxygen, rather than nitrogen, is the preferred site for solvent-to-solute hydrogen bonding in the covalent isocyanates.}, issn = {0022-2860}, doi = {http://dx.doi.org/10.1016/0022-2860(94)08385-1}, url = {http://www.sciencedirect.com/science/article/pii/0022286094083851}, author = {Michal Witanowski and Zenobia Biedrzycka and Wanda Sicinska and Graham A. Webb} } @article {320, title = {A Study of Solvent Effects on the NMR Shieldings of the Two Types of Nitrogen Atoms Contained in Diazole Ring Systems}, journal = {Journal of Magnetic Resonance, Series A}, volume = {109}, year = {1994}, pages = {177 - 180}, abstract = {The diazole ring systems, N-methyl derivatives of pyrazole and imidazole, show a clear differentiation between pyrrole and pyridine types of nitrogen atoms from the point of view of their solvent-induced nitrogen shielding variations. The largest effects on the nitrogen shieldings derive from hydrogen bonding from solvent to solute, which involve the lone pair electrons at the pyridine-type nitrogen atoms of the solute. These effects may be used to identify sites of hydrogen-bond accepters in the solute molecules. Solvent-polarity effects on the nitrogen shieldings suggest a significant charge migration from pyrrole- to pyridine-type nitrogen atoms in such systems. Solvaton calculations amply support this proposal by correctly predicting the observed directions and relative magnitudes of the nitrogen shielding variations as a function of solvent polarity.}, issn = {1064-1858}, doi = {http://dx.doi.org/10.1006/jmra.1994.1152}, url = {http://www.sciencedirect.com/science/article/pii/S1064185884711521}, author = {Michal Witanowski and Wanda Sicinska and Zenobia Biedrzycka and Graham A. Webb} } @article {324, title = {Solvent versus substituent effects on the nitrogen NMR shielding of the nitro-group in substituted benzenes}, journal = {Magnetic Resonance in Chemistry}, volume = {31}, year = {1993}, pages = {916{\textendash}919}, publisher = {John Wiley \& Sons, Ltd.}, abstract = {Intermolecular effects are shown to induce a variation of about 5 ppm in the nitrogen NMR shieldings of aromatic nitro groups. The latter turn out to be comparable to those exerted by substituents in nitrobenzene derivatives in solutions in a given solvent. Substituent effects on the NO2 nitrogen shielding in nitro benzenes, in a given solvent, seem to reflect contributions from the so-called field-inductive effects on the electron charge distribution in such systems, and are insensitive to the resonance effects concerned with the delocalized pπ-electron systems involved.}, keywords = {14N shielding, Solvent and substituent effects, Substituted nitrobenzenes}, issn = {1097-458X}, doi = {10.1002/mrc.1260311008}, url = {http://dx.doi.org/10.1002/mrc.1260311008}, author = {Michal Witanowski and Wanda Sicinska and Zenobia Biedrzycka and Graham A. Webb} }